Method and apparatus manufacturing hot press formed parts for multi-step process

ABSTRACT

Provided are a method and apparatus for manufacturing hot press formed parts for a multi-step process, the method comprising: a heating step for heating a strip material; a transferring step for transferring the heated strip material to a processing apparatus having mounted on a press a plurality of molds comprising a notching mold and/or a blanking mold, a forming mold, and a trimming mold; a notching step, for obtaining a notched material connected to the strip by means of a web portion by cutting a part of the material by means of the notching mold, and/or a blanking step for obtaining a blanked material separated from the strip by cutting a part of the material by means of the blanking mold; a forming step of transferring and positioning the material which has gone through the notching step and/or the blanking step near the forming mold, and then forming the material by means of the forming mold; and a trimming step for removing, by means of the trimming mold, an outer edge portion of the material which is unnecessary for a final product shape.

TECHNICAL FIELD

The present disclosure relates to a method and apparatus formanufacturing a hot press formed member for a multistage process havingone or more improved properties of productivity and formability.

BACKGROUND ART

In accordance with demand for weight reduction and improvements in thesafety of vehicles, high-strength steel using a hot press forming methodhas been actively applied. In general, in the hot press forming method,a blank having a shape designed to form a part is prepared, heated to ahigh temperature, and then put into a mold mounted on a press.Thereafter, a pressing slide is lowered to bottom dead center to form ablank, and a formed part is rapidly cooled in the mold by holding thepressing slide at bottom dead center for a predetermined amount of time.As such, after sufficiently cooling the formed part, a part ismanufactured by a process of lifting the slide and removing a product.In addition, in general, the part manufactured as described above goesthrough a procedure of being manufactured into a final part shape byadditionally performing a process of cutting some unnecessary edgeportions or internal hole portions from the final part shape. Theprocess of cutting the product after forming is performed by lasertrimming or mechanical trimming using a trimming mold/press because theformed product has high strength.

The general hot press forming described above has a limitation in that afinal part should be manufactured by single forming. Therefore, there isa disadvantage in that it is not possible to manufacture a part shapethat is difficult to be manufactured using single forming. In order tosolve this problem, a method of utilizing indirect hot press forming inwhich a part is formed into an intermediate shape at room temperatureand the preformed part is heated and then manufactured into a final partshape through hot press forming has been also partially applied.However, such indirect hot press forming has a disadvantage in thatvarious additional production costs are required for an additionalprocess of forming apart into an intermediate shape and a specialheating furnace capable of heating the formed part.

On the other hand, in general cold press forming, it is a commonapproach to forma final part through a forming process with severalsteps. As such a cold press forming method, a method of forming a partby dividing a process into several processes using several presses andmolds or forming a part by putting a multistage mold into one press hasbeen used. Meanwhile, as a method of forming a part by putting amultistage mold into one press, a transfer type forming method in whichblanks separated from each other are put into a mold and formed inmultiple steps or a progressive type forming method in which a materialin the form of a strip that is continuously connected is supplied andformed in multiple steps has been generally used.

However, unlike the cold press forming, it is not easy to apply aforming method contains several steps to the hot press forming becausethe hot press forming has a special restriction that high strengthshould be secured through rapid cooling of a high-temperature material.

(Patent Document 1) Korean Patent Laid-Open Publication No. 2006-0054479

DISCLOSURE Technical Problem

An aspect of the present disclosure is to provide a method and apparatusfor manufacturing a hot press formed member for a multistage processhaving one or more improved properties of productivity and formability.

Another aspect of the present disclosure is to provide a formed memberhaving excellent strength characteristics while having a complex shapethat has not been obtained in a hot press forming method with a singleprocess according to the related art.

An object of the present disclosure is not limited to the abovedescription. Those skilled in the art to which the present disclosurepertains will have no difficulties in understanding additional objectsof the present disclosure from the general contents of the specificationof the present disclosure.

Technical Solution

According to an aspect of the present disclosure, a method formanufacturing a hot press formed member in a multistage processincludes:

a heating step of heating a strip material;

a transfer step of transferring the heated strip material to aprocessing apparatus in which a plurality of molds including one or moremolds of a notching mold and a blanking mold; a forming mold; and atrimming mold; are mounted on one press;

one or more steps of a notching step of obtaining a notched materialconnected to the strip by a web portion by cutting a part of thematerial using the notching mold and a blanking step of obtaining ablank material separated from the strip by cutting a part of thematerial using the blanking mold;

a forming step of transferring the material subjected to one or moresteps of the notching step and the blanking step and positioning thematerial in the vicinity of the forming mold, and then forming thematerial using the forming mold; and

a trimming step of removing an unnecessary outer edge portion of thematerial from a final product shape using the trimming mold.

According to another aspect of the present disclosure, an apparatus formanufacturing a hot press formed member for a multistage processincludes:

a supply unit for continuously supplying a strip material;

a heating unit for heating the strip material;

a processing unit including a processing apparatus in which a pluralityof molds including one or more molds of a notching mold and a blankingmold, a forming mold, and a trimming mold are mounted on one press; and

a transfer unit for transferring the strip material heated in theheating unit to the processing unit.

According to still another aspect of the present disclosure,

there is provided a hot press formed member manufactured by the methodfor manufacturing a hot press formed member for a multistage process,

wherein the hot press formed member has an under-cut shape, and atensile strength of the hot press formed member is 1, 300 MPa or more.

Advantageous Effects

As set forth above, according to an aspect of the present disclosure,the strip material may be stably heated and supplied with a fast cycletime, such that it is possible to provide a method and apparatus formanufacturing a hot press formed member for a multistage process havingimproved productivity.

Further, according to another aspect of the present disclosure, thestrip material is formed by a multistage process, such that it ispossible to relatively easily provide a complicated shape in comparisonto hot press forming with a single process.

Further, according to still another aspect of the present disclosure, itis possible to provide a formed member having excellent strengthcharacteristics while having a complex shape that has not been obtainedin a hot press forming method with a single process according to therelated art.

The various and beneficial advantages and effects of the presentdisclosure are not limited to the above description, and may be moreeasily understood in the description of specific exemplary embodimentsin the present disclosure.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an exemplary hot press forming apparatus for amultistage process of the present disclosure.

FIG. 2A illustrates a result of a temperature distribution duringresistance heating for a material having a uniform width, and FIG. 2Billustrates a result of a temperature distribution during resistanceheating for a material having a non-uniform width.

FIG. 3 is a graph showing an operating method of an exemplary mechanicalpress of the present disclosure in a relationship of a stroke to a crankangle.

FIG. 4 is a graph showing a temperature change of a material over timein a case in which a common progressive method is applied in anexemplary notching step of the present disclosure.

FIG. 5 is a graph showing a temperature change of a material over timein a case in which a notching upper mold and a notching lower mold arebrought into contact with a strip material only in a cutting process inan exemplary notching step of the present disclosure.

FIGS. 6 through 8 are graphs showing a temperature change of a materialover time according to Examples 14 to 16, respectively.

FIG. 9 illustrates a change in thickness reduction rate in a case inwhich an exemplary forming process of the present disclosure isperformed in a single process.

FIG. 10 illustrates a change in thickness reduction rate in a case inwhich an exemplary forming process of the present disclosure isperformed by being divided into two steps.

FIG. 11 illustrates an exemplary hot press forming method for amultistage process of the present disclosure.

FIG. 12 illustrates a change of a material during performance of anexemplary multistage process of the present disclosure.

FIG. 13 illustrates an exemplary notching or blanking method of thepresent disclosure.

FIG. 14 illustrates an exemplary notching or blanking method using apush bar of the present disclosure.

FIGS. 15 through 17 illustrate a method of increasing contact time of anexemplary mold of the present disclosure.

FIG. 18 illustrates a structure of an exemplary hot press formed memberof the present disclosure, in which FIG. 18A illustrates a perspectiveview of the formed member, and FIG. 18B illustrates a side view of theformed member.

BEST MODE FOR INVENTION

In hot press forming, it is difficult to apply a forming method that hasundergone several steps because high strength should be secured byrapidly cooling a high-temperature material. Accordingly, in recentyears, there has been studied a method of applying a process of heatinga blank using a galvanized steel material for hot press forming, puttingthe blank into a mold for several steps mounted on a mechanical servopress, and then applying a process of rapidly cooling the blank to arelatively low temperature, a process of piercing the blank, and aprocess of finally trimming the blank. However, in this method, it isessential to secure a sufficient performance of a heating furnace tosupply the blank heated at a rate required by the press.

For example, a case in which the press is operated at a rate of 15 SPM(15 strokes per minute) means that there is a need for a heating furnacethat may supply the heated blank every 4 seconds. Since a cycle time ofcommon hot press forming is also on the order of 16 seconds, it may beconsidered that the operation is performed fairly quickly. It may beseen from this that there is a need for a performance of a heatingfurnace that may supply the heated blank every 16 seconds.

However, when the cycle time is 4 seconds, it is required to supply theblank heated at a rate 4 times faster than that in the case in which thecycle time is 16 seconds as described above. When the material stays inthe heating furnace for a required time, the material is heated to adesired temperature, which means that 4 times as many blanks should beput into the heating furnace in order to heat the blank at a rate 4times faster. Therefore, it may be presumed that a length of the heatingfurnace should be arithmetically longer by 4 times. For example, alength of the common heating furnace is about 30 m, and as describedabove, in order to heat the blank at a rate 4 times faster, the lengthof the heating furnace should be at a level of 120 m, or four heatingfurnaces having a length of m should be installed in parallel.

However, an increase in length of the heating furnace becomes a bigobstacle in terms of space and cost. Therefore, a facility capable ofrapidly heating the blank may be used instead of using the heatingfurnace in a general atmosphere described above.

As a result of an earnest examination, the present inventors have foundthat it is not technically easy to rapidly and uniformly heat a blankhaving a shape. Accordingly, the present inventors have continuedstudies and have invented a hot press forming apparatus and method for amultistage process that implement a stable supply of a high-temperaturematerial at a high rate capable of responding to a short cycle time andalso implement improvement of formability.

In addition, the general hot press forming includes a process ofperforming cooling while holding the press at a bottom dead center for apredetermined time after forming and maintaining the mold in a closedstate. However, in the case of the multistage process, since processesof several steps should be sequentially performed, cooling is notperformed only in one mold, but is performed in a multistage mold.Therefore, in the hot press forming method for a multistage process ofthe present disclosure, it is required to design a process consideringthe effect of various process factors such as a press rate and thenumber of processes on cooling. Accordingly, the present inventors havedesigned an optimal multistage process in consideration of variousprocess factors.

[Method for Manufacturing Hot Press Formed Member for MultistageProcess]

An aspect of the present disclosure provides a method for manufacturinga hot press formed member for a multistage process, the methodincluding:

a heating step of heating a strip material;

a transfer step of transferring the heated strip material to aprocessing apparatus in which a plurality of molds including one or moremolds of a notching mold and a blanking mold, a forming mold, and atrimming mold are mounted on one press;

one or more steps of a notching step of obtaining a notched materialconnected to the strip by a web portion by cutting a part of thematerial using the notching mold and a blanking step of obtaining ablank material separated from the strip by cutting a part of thematerial using the blanking mold;

a forming step of transferring the material subjected to one or moresteps of the notching step and the blanking step and positioning thematerial in the vicinity of the forming mold, and then forming thematerial using the forming mold; and

a trimming step of removing an unnecessary outer edge portion of thematerial from a final product shape using the trimming mold.

According to an aspect of the present disclosure, the method may includethe heating step of heating the strip material, and the strip materialmay be heated in a temperature range of 850 to 960° C. According to anaspect of the present disclosure, rapid heating may be applied as theheating method, and a heating rate may be in a range of 12 to 200° C./s.Meanwhile, a lower limit of the heating rate may be preferably 13° C./s,and the lower limit of the heating rate may be more preferably 30° C./s.In addition, an upper limit of the heating rate may be 100° C./s, andthe upper limit of the heating rate may be more preferably 80° C./s. Inaddition, the heating temperature may be a value measured based on atemperature change based on any one point on a surface of the stripmaterial.

According to an aspect of the present disclosure, as the rapid heatingmethod, various methods such as induction heating, resistance heating,and infrared heating may be used. A strip material having a uniformwidth is heated using the rapid heating method described above, suchthat it is possible to secure a more uniform temperature distribution incomparison to a method of heating a blank material having a shapeaccording to the related art, which ensures a stable supply of theheated strip material with a fast cycle time.

On the other hand, in the method of heating the blank material accordingto the related art, since it is difficult to perform uniform heating, anatmospheric heating furnace is mostly used. However, in order to supplythe heated blank with a short cycle time of a multistage process, thereis a problem in that the length of the heating furnace is excessivelyincreased. Accordingly, in the present disclosure, it has been foundthat the problem described above can be solved by applying a rapidheating method to the strip material itself having a uniform width.

For example, in a case in which the cycle time is 4 seconds (in thiscase, a pitch amount designed in the multistage process to be describedbelow is based on 500 mm), when a method of rapidly heating the blank toa final temperature in only 16 seconds is used, the length of the stripmaterial involved in the heating process is calculated at a level of 2m. Therefore, it is expected that a heating space may be remarkablyreduced in comparison to a parallel arrangement method of four heatingfurnaces having a length of 120 m or 30 m in the method of heating theblank according to the related art described above as an example.

That is, according to an aspect of the present disclosure, a ratio (L/S)of the total effective length (L) of the heating devices to the cycletime (S) may be in a range of 0.25 to 4, but is not particularly limitedthereto. When the range described above is satisfied, it is possible toprovide a hot press forming method that may implement a stable supply ofa material with a fast cycle time and improvement of formability.

In this case, the total effective length (L) of the heating devicesrefers to a length of a part directly used for heating the material, andm is used for a unit thereof. As an example, in the case of the heatingfurnace, an effective length may refer to an internal length of theheating furnace, and in the case of the induction heating, an effectivelength may refer to a length of a radio frequency coil used for heating.That is, the effective length refers to the total effective length ofall the heating devices even in a case of including one or more ofseries type heating and parallel type heating. Specifically, when fourheating furnaces having a length of 30 m are arranged in parallel, thetotal effective length of the heating devices is 120 m [=30 m×4]. Inaddition, the cycle time described above refers to a time during whichone cycle is performed in a multistage process, and seconds (sec) isused for a unit thereof.

Meanwhile, according to an aspect of the present disclosure, as thestrip material, a plated steel sheet including a base steel sheet and aplating layer provided on the base steel sheet may be used. That is,when the rapid heating method is applied to the strip material, analuminum plated steel sheet or an aluminum alloy plated steel sheet maybe applied in order to achieve sufficient alloying of the plating layerand to prevent melting and volatilization of the plating layer.

According to an aspect of the present disclosure, the base steel sheetmay have a composition containing, by wt %, 0.1 to 0.5% of C, 0.1 to 2%of Si, 0.5 to 3% of Mn, 0.01 to 0.5% of Cr, 0.001 to 1.0% of Al, 0.05%or less of P, 0.02% or less of S, 0.02% or less of N, 0.002 to 0.005% ofB, and a balance of Fe and other unavoidable impurities, but the basesteel sheet is not limited to this composition. In addition, the platinglayer may be alloyed with a plating material produced by hot-dip platingso as to have a composition containing, by wt %, 5 to 11% of Si, 4.5% orless of Fe, and a balance of Al and other unavoidable impurities, butthe plating layer is not limited to this composition. That is, as anon-limiting example of the present disclosure, an aluminum plated steelsheet or an aluminum alloy plated steel sheet satisfying the compositiondescribed above may be used.

According to an aspect of the present disclosure, as the strip material,an aluminum plated steel sheet or an aluminum alloy plated steel sheethaving an Fe content of 5 wt % or more and preferably 5 to 60 wt % (morepreferably, 30 to 60 wt %) in a surface portion of the plating layer maybe used. In this case, the surface portion of the plating layer refersto a region within 2 μm from the surface of the plating layer.Meanwhile, a material in which an Fe content in the surface portion ofthe plating layer satisfies the range described above is used as thestrip material, such that high-temperature heating may be performed in astate in which a sufficient alloying time is secured when rapid heatingis applied.

According to an aspect of the present disclosure, the method may furtherinclude, before the heating step, a step of continuously supplying theprepared strip material to the heating unit. In the supply of the stripmaterial, the material prepared in the form of a coil may becontinuously supplied to the heating unit in the form of a stripmaterial having a uniform width by an uncoiler. In this case, the stripmaterial having a uniform width means that the width of the stripmaterial measured in a direction perpendicular to a transfer directionof the material is uniform based on the surface of the strip material.

According to an aspect of the present disclosure, the “continuoussupply” may mean that the strip material is supplied to the heating unitat a constant rate, except for a moment when the supply is temporarilystopped in consideration of a subsequent process.

According to an aspect of the present disclosure, the method includesthe transfer step of transferring the heated strip material to theprocessing apparatus in which a plurality of molds including one or moremolds of a notching mold and a blanking mold, a forming mold, and atrimming mold are mounted on one press.

According to an aspect of the present disclosure, the heated stripmaterial may be continuously transferred to the processing apparatus. Inthis case, as for the meaning of the continuous transfer, the samedescription as that of the “continuous supply” as described above may beapplied, except that the heated strip material is transferred to theprocessing apparatus.

According to an aspect of the present disclosure, it is preferable thatthe cooling of the heated strip material is minimized in the transferstep described above, and thus, the transfer step of the strip materialmay be performed in a thermal insulation chamber. That is, the stripmaterial is transferred in the thermal insulation chamber describedabove, such that heat loss prevention and easy processability in theprocessing apparatus may be secured.

According to an aspect of the present disclosure, the thermal insulationchamber may be maintained within a temperature range of Ts−200° C. orhigher and Ts+50° C. or lower based on a surface temperature (Ts) of thestrip material supplied to the thermal insulation chamber. As such, whenthe temperature of the thermal insulation chamber is controlled toTs−200° C. or higher, the cooling of the strip material may be minimizedduring the transfer, and easy processability in the processing apparatusmay also be secured, and when the temperature of the thermal insulationchamber is controlled to Ts+50° C. or lower, it is possible to preventan additional increase in temperature during the transfer. Meanwhile, interms of further improving the easy processability and the effect ofpreventing the occurrence of the additional increase in temperatureduring the transfer described above, the temperature of the thermalinsulation chamber may be preferably 900° C. or higher and Ts+50° C. orlower, and may be more preferably in a range of 900 to 1,010° C.

In this case, the heating temperature range of the material generallyused in the art may also be applied to the surface temperature (Ts) ofthe strip material supplied to the thermal insulation chamber, and thusthe range is not particularly limited in the present specification.However, as a non-limiting example, the surface temperature (Ts) of thestrip material may be in a range of 850 to 960° C.

Meanwhile, according to an aspect of the present disclosure, on the onepress, one or more molds of the notching mold and the blanking mold andthe forming mold may be mounted to be spaced apart from each other by adesigned pitch amount in the transfer direction of the strip material.For example, in a case in which only the nothing mold (or only theblanking mold) is included, on the one press, the notching mold (or theblanking mold) is sequentially mounted in advance in the transferdirection of the strip material, and then the forming mold may bemounted to be spaced apart from the notching mold (or the blanking mold)by a designed pitch amount. Alternatively, in a case in which both thenotching mold and the blanking mold are included, on the one press, thenotching mold, the blanking mold, and the forming mold may besequentially mounted in the transfer direction of the strip material,and in this case, two molds adjacent to each other in the transferdirection may be spaced apart from each other by a designed pitchamount.

In addition, according to an aspect of the present disclosure, on theone press, the forming mold and the trimming mold may be mounted to bespaced apart from each other by a designed pitch amount in the transferdirection of the strip material. That is, on the one press, the notchingmold (and/or the blanking mold) and the forming mold may be sequentiallymounted in the transfer direction of the strip material in the samemanner as described above, and the trimming mold may be mounted to bespaced apart from the forming mold by a designed pitch amount.

Alternatively, as described below, in a case in which one or more stepsof a piercing step and a flanging step are further included, on the onepress, a piercing mold (and/or a flanging mold) may be further mountedbetween the forming mold and the trimming mold in the transfer directionof the material. For example, in a case in which a piercing step isfurther included, on the one press, the notching mold (and/or theblanking mold), the forming mold, the piercing mold, and the trimmingmold may be sequentially mounted in the transfer direction of thematerial.

In addition, according to an aspect of the present disclosure, inaddition to the piercing step and the flanging step described above, anadditional processing step commonly applicable in the art may be furtherincluded according to the object of the present disclosure. Accordingly,an additional processing mold may be mounted on the one press accordingto the purpose to correspond to the additional process step describedabove.

In this case, according to an aspect of the present disclosure, in theone press, two molds adjacent to each other in the transfer direction ofthe strip material may be arranged to be spaced apart from each other bya designed pitch amount. For example, in a case in which the piercingstep is further included, on the one press, the piercing mold may bemounted to be spaced apart from the forming mold by a designed pitchamount, and the trimming mold may be mounted to be spaced apart from thepiercing mold by a designed pitch amount.

Alternatively, according to an aspect of the present disclosure, asdescribed below, in a case in which one or more of one or more steps ofthe notching step and the blanking step; the forming step; one or moresteps of the piercing step and the flanging step (however, the piercingstep and the flanging step may be omitted); and the trimming step areperformed in a multistage process divided into two or more steps, aplurality of molds provided for the step performed by being divided intoa multistage process may be mounted on one press to be spaced apart fromeach other by a designed pitch amount in the transfer direction of thestrip material. In this case, on the one press, the first mold in thestep performed in a multistage process may be mounted to be spaced apartfrom the mold in the previous step by a designed pitch amount in thetransfer direction of the strip material. In addition, the last mold inthe step performed in a multistage process may be mounted to be spacedapart from the mold in the immediately following step by a designedpitch amount in the transfer direction of the strip material.

For example, in a case in which the forming step is performed in amultistage process with two steps, in one press, a primary forming moldand a secondary forming mold may be mounted to be spaced apart from eachother by a designed pitch amount in the transfer direction of the stripmaterial. In this case, the primary forming mold may be mounted on onepress to be spaced apart from the notching mold (in the case ofincluding the notching step) in the previous step by a designed pitchamount in the transfer direction of the strip material. In addition, thesecondary forming mold that is the last mold in the forming stepperformed in a multistage process may be mounted on one press to bespaced apart from the piercing mold (in the case of including thepiercing step) used in the immediately following step by a designedpitch amount in the transfer direction of the strip material.

According to an aspect of the present disclosure, as for the pluralityof molds, pitch amounts designed between two molds adjacent to eachother in the transfer direction of the strip material may be the same aseach other. Meanwhile, FIG. 1 illustrates a transfer direction X of thestrip material, and illustrates pitch amounts 50 designed between twomolds adjacent to each other in the transfer direction X. The designedpitch amount may refer to a distance between the molds measured based onthe center of each mold (a point that is ½ of the length of one mold inthe transfer direction of the material).

According to an aspect of the present disclosure, all of the pluralityof molds mounted on one press may be moved in conjunction with apressing motion by the one press. That is, an upper mold and a lowermold for each mold mounted on the one press may be combined inconjunction with the pressing motion by the one press described above.That is, when a pressing slide is lowered and stays at a pressing bottomdead center, a combination of each mold mounted on the press may beperformed.

Therefore, as described above, the pitch amounts designed for two moldsadjacent to each other in the transfer direction of the material arecontrolled to be the same, such that when the processing of the materialis performed by a progressive method corresponding to an example of thepresent disclosure, the transfer of the material between the molds maybe controlled consistently. Through this, it is possible to secureimprovement in productivity and processability and to easily perform amultistage process.

That is, according to an aspect of the present disclosure, after eachclosing of a press upper plate (pressing slide) and a press lower plate(press bolster) according to the pressing motion of the one press isperformed once, the material may be transferred in the transferdirection of the material by a designed pitch amount.

According to an aspect of the present disclosure, the method may includeone or more steps of the notching step of obtaining a notched materialconnected to the strip by a web portion by cutting a part of thematerial using the notching mold and the blanking step of obtaining ablank material separated from the strip by cutting a part of thematerial using the blanking mold.

In the notching step or the blanking step, the upper mold and the lowermold of each mold are combined, such that the material may be processedinto a desired shape by preliminarily removing an unnecessary portionfrom the material. In this case, the material processed into a desiredshape obtained through the notching step may be a notched materialconnected to the strip by the web portion. In addition, the materialprocessed into a desired shape obtained through the blanking step may bea blank material separated from the strip.

According to an aspect of the present disclosure, a heated mold may beused in one or more steps of the notching step and the blanking step,and although not particularly limited, an initial temperature of one ormore molds of the notching mold and the blanking mold may be 50° C. orhigher. Meanwhile, the higher the temperature of the mold, the betterthe effect of suppressing cooling, and thus, an upper limit of theinitial temperature of the mold is not particularly limited. However,the upper limit of the initial temperature of the mold may be preferablyequal to or lower than the heating temperature of the strip material.That is, a heated notching mold may be used in the notching step and/ora heated blanking mold may be used in the blanking step. In general, inthe notching step and the blanking step, it is essential that the uppermold and the lower mold of the mold are brought into contact with eachother in order to process the material. Therefore, the heated mold isused in the notching step and the blanking step, such that it ispossible to prevent the strip material heated to a high temperature fromcoming into contact with a cold mold, and thus, it is possible tosuppress rapid cooling of the material and to secure easy formability ina subsequent process.

According to an aspect of the present disclosure, in order to suppresscooling of the strip material, one or more initial temperatures of thenotching mold and the blanking mold may be 400° C. or higher and morepreferably 500° C. or higher. The initial temperature of the mold refersto an initial temperature of the mold at a point in time when thematerial is put into the mold. Meanwhile, the higher the temperature ofthe mold, the better the effect of suppressing cooling, and thus, anupper limit of the initial temperature of the mold is not particularlylimited. However, the upper limit of the initial temperature of the moldmay be preferably equal to or lower than the heating temperature of thestrip material. As described above, the initial temperature of the moldis controlled to be 400° C. or higher, such that it is possible tosecure easy formability in the forming step, which is a subsequentprocess. In addition, the initial temperature of each of the notchingand blanking molds is controlled to be equal to or lower than theheating temperature of the strip material, such that it is possible toprevent an additional increase in temperature due to contact duringnotching and blanking.

According to an aspect of the present disclosure, a surface temperatureof the material immediately after one or more steps of the notching stepand the blanking step may be controlled to 700° C. or higher and may bemore preferably controlled to 712° C. or higher. Meanwhile, an upperlimit of the surface temperature is not particularly limited, but may bepreferably equal to or lower than the heating temperature of the stripmaterial. In this case, the surface temperature of the material is avalue measured based on when the contact between the mold and thematerial is finished immediately after one or more steps of the notchingstep and the blanking step. Meanwhile, the surface temperature of thematerial satisfies the range described above, such that excellentformability and prevention of an additional increase in temperature maybe simultaneously achieved in a subsequent process.

Meanwhile, as illustrated in FIGS. 11 through 13 , in a case of ageneral progressive method, a strip material 200 transferred to aprocessing apparatus 41 is generally maintained at a transfer heightlevel 110 a of the material and is then lowered according to a movementof an upper mold 61 a in a contact state with the notching upper mold 61a as contact with the notching upper mold 61 a occurs. As such, thestrip material 200 is lowered in the contact state with the notchingupper mold 61 a and is then notched by cutting a part of the stripmaterial 200 as the strip material also comes into contact with anotching lower mold 61 b in the vicinity of a bottom dead center of theslide 110 b. Subsequently, the notched material is lifted again up tothe original transfer height level while coming into contact with thenotching upper mold 61 a.

Referring to FIG. 4 illustrating an exemplary experimental resultreflecting such matters, it may be confirmed that the temperature of thematerial is rapidly decreased in a section where the contact of thenotching upper mold is started. Accordingly, the present inventors havefurther found that when the strip material comes into contact with themold only at a point in time when the strip material is cut, the timeduring which the strip material comes into contact with the upper moldof the mold may be minimized to reduce cooling. In addition, althoughnot additionally illustrated in the drawing, in the same manner as inthe notching step described above, even in the blanking step, coolingmay be reduced by minimizing the time during which the material comesinto contact with the upper mold of the blanking mold.

That is, according to an aspect of the present disclosure, in thenotching step and the blanking step, the strip material put into theprocessing apparatus may be controlled so that the strip material islowered to the pressing bottom dead center from the transfer heightlevel of the material in a non-contact state with the upper mold of themold, and then comes into contact with the upper mold (or an upper moldsurface) and the lower mold (or a lower mold surface) of the mold onlyin the cutting process.

For example, in a case in which the notching step or the blanking stepis included, in the notching step (or the blanking step), the stripmaterial put into the processing apparatus may be controlled so that thestrip material is lowered to the pressing bottom dead center from thetransfer height level of the material in a non-contact state with thenotching upper mold (or the blanking upper mold), and then comes intocontact with the notching upper mold and the notching lower mold (or theblanking upper mold and the blanking lower mold) only in the cuttingprocess.

Alternatively, in a case in which both the notching step and theblanking step are included, the notching step is performed in the samemanner as described above, and then the blanking step may be performedin the same manner as described above except that it is premised on anotched material.

In addition, according to an aspect of the present disclosure, in thenotching step (or the blanking step), when the strip material is liftedafter the cutting process, as in the lowering described above, the stripmaterial may be separated from the notching (or blanking) lower mold ina non-contact state with the notching (or blanking) upper mold and thenthe strip material may be lifted to the transfer height level of thestrip material. As such, in the notching step (or the blanking step),the notching (or blanking) upper mold and the notching (or blanking)lower mold are controlled so that they come into contact with the stripmaterial only at a point of cutting of the strip material, such thatexcessive cooling in the notching step (or the blanking step) may besuppressed, thereby securing easy formability in the subsequent formingstep.

Meanwhile, in FIG. 1 illustrating an exemplary hot press formingapparatus for a multistage process of the present disclosure, among thenotching mold and the blanking mold, in a case in which only thenotching mold is included, the notching upper mold 61 a and the notchinglower mold 61 b are illustrated, and in a case in which only theblanking mold is included, the blanking upper mold 61 a and the blankinglower mold 61 b are illustrated. However, a drawing illustrating a casein which both the notching mold and the blanking mold are included isomitted.

In the notching step and the blanking step described above, variousmethods may be used as a method in which the strip material comes intocontact with the upper mold and the lower mold of the mold only in thecutting process. For example, as illustrated in FIG. 14 , as a structureprotruding from a surface of the notching upper mold (or the blankingupper mold) in contact with the strip material, a push bar 11 operatedby a spring may be provided. In this case, the push bar may bepositioned on the surface of the notching upper mold (or the blankingupper mold) in contact with the strip material so as to correspond tothe portion to be removed from the material in a subsequent process.

That is, according to an aspect of the present disclosure, the push bar11 may allow the notching upper mold (or the blanking upper mold) to belowered and lifted while pressing the material 200 to be in contact withthe portion to be removed from the material 200 in a subsequent process.Through this, when the press is lowered and lifted, the press may becontrolled so as not to be in contact with the strip material in aregion other than the region where the push bar is provided on thesurface of the notching upper mold (or the blanking upper mold) incontact with the strip material. As an example, the portion to beremoved from the strip material in a subsequent process may correspondto a guide bar, a guide pin, or the like that guides the strip fortransferring the strip.

Meanwhile, according to an aspect of the present disclosure, the presentinventors have further found that, during the pressing motion of theprocessing apparatus, as the time for the material to stay in thevicinity of the pressing bottom dead center (that is, the holding timein the vicinity of the pressing bottom dead center) is increased, theclosing time of the upper mold and the lower mold of the mold isincreased, and accordingly, the cooling rate of the material may beincreased.

FIG. 3 illustrates a stroke of the slide according to a crank angle foran operating method of an exemplary mechanical pressing motion of thepresent disclosure. FIG. 3A corresponds to a general crank motionmethod, and FIG. 3B corresponds to other motion methods such as link,knuckle, and servo motion methods. It may be confirmed that there is adifference in the holding time of the slide in the vicinity of thebottom dead center according to the difference in the method describedabove, and in the case of the motion method of FIG. 3B, a percentage ofa holding time in the vicinity of the bottom dead center is higher thanthat of the motion method of FIG. 3A.

Therefore, according to an aspect of the present disclosure, thepressing motion of the processing apparatus may be performed by any onemethod selected from the group consisting of a link motion method, aknuckle motion method, and a servo motion method, but is notparticularly limited thereto.

Alternatively, according to an aspect of the present disclosure, in thepressing motion of the processing apparatus, a percentage of a holdingtime in the vicinity of the bottom dead center in one stroke may be 4 to30% and more preferably to 30%. In this case, the percentage of theholding time in the vicinity of the bottom dead center refers to apercentage of the time the press to stay to a point corresponding to 1mm from the pressing bottom dead center in an upward direction.

According to an aspect of the present disclosure, when the percentage ofthe holding time is 4% or more, a minimum mold contact time for coolingis secured, such that an increase in the number of processes forsecuring physical properties may be prevented. In addition, throughthis, it is possible to prevent an increase in extraction time of thefinal part so as to secure a critical cooling rate, such that desiredphysical properties may be easily secured. In addition, when thepercentage of the holding time is 30% or less, an unnecessary holdingtime in the vicinity of the bottom dead center is reduced, such that itis possible to secure a stable process by sufficiently securing theprocessing time for the up-and-down motion of the pressing slide and thetransfer of the strip.

According to an aspect of the present disclosure, one or more steps ofthe notching step and the blanking step may be performed in a singleprocess, and may be performed in a multistage process by being dividedinto two or more steps.

In addition, according to an aspect of the present disclosure, themethod may include the forming step of transferring the materialsubjected to one or more steps of the notching step and the blankingstep and positioning the material in the vicinity of the forming mold,and then forming the material using the forming mold (that is, bycombining the upper mold and the lower mold of the forming mold).

According to an aspect of the present disclosure, the forming step maybe performed in a single process, or may be performed in a multistageprocess by being divided into two or more steps.

According to an aspect of the present disclosure, although notparticularly limited, in the forming step described above, the formingstep may be performed in a multistage process with two or more steps toform an under-cut shape (the same contents to be described below areapplied to the under-cut shape).

According to an aspect of the present disclosure, in a case in which theforming step described above is performed in a multistage process withtwo or more steps, a forming direction of the material in one formingstep and a forming direction of the material in the other forming stepmay be different from each other. For example, a forming direction ofthe material in a primary forming step and a forming direction of thematerial in an additional forming step of forming an under-cut shapeafter the primary forming step may be different from each other. In thiscase, the fact that the forming directions are different from each othermeans that the forming directions are not parallel to each other.

As a non-limiting example of the present disclosure, in the primaryforming step, a burring portion may be first formed in a verticalforming direction based on the surface of the material, and then bendingprocessing may be performed on the material in the forming step afterthe primary forming step (for example, the secondary forming step). Inthis case, the bending processing is performed to form an angle of 90°or less with the forming direction described above, such that theforming direction in the primary forming step and the forming directionsin the second and subsequent forming steps may be different from eachother.

In this case, according to an aspect of the present disclosure, afterthe bending processing is performed only once, the burring portion maynot be cooled by additional mold contact. Therefore, when the burringportion is formed in the forming step, the process may be designed toperform the bending processing after cooling is sufficiently performed.

That is, according to an aspect of the present disclosure, in theprimary forming step, processing may be performed on the material toform a formed portion such as a burring portion, and bending processingmay be performed on the material in a forming step after the primaryforming step, thereby forming a bent portion (that is, a portion formedby connecting a flat portion and a side portion) to be described belowon the material.

Meanwhile, as another non-limiting example of the present disclosure,the material may be formed in the primary forming step based on theplane of the material using the forming mold so that the materialincludes a flat portion (A) having a first plane direction (S) and aside portion (B) having a second plane direction (P) different from thefirst plane direction. Subsequently, in the forming step after theprimary forming step (for example, the secondary forming step), thematerial may be formed by applying pressure to the side portion toinclude a portion in which an angle (D) formed by the first planedirection (S) and the second plane direction (P) is within 90° or less.

According to an aspect of the present disclosure, the method includesthe forming step performed in a process with two or more steps in whichthe forming directions are different from each other as described above,such that it is possible to provide a hot press formed member having anunder-cut shape to be described below.

That is, the method includes the forming step performed in a multistageprocess with two or more steps according to an aspect of the presentdisclosure, such that it is possible to easily form an under-cut shapethat has not been formed by the prior art in which the forming step isperformed in a single process. Therefore, according to the presentdisclosure, a formed member having an overall tensile strength of 1,300MPa or more may be easily manufactured by applying a hot press formingmethod even when the formed member has the under-cut shape describedabove as a final product.

According to an aspect of the present disclosure, in a case in which thenotching step or the blanking step is included, when the material istransferred from the notching mold or the blanking mold to the vicinityof the forming mold, the notched material (or the blank material) may betransferred by a designed pitch amount to correspond to the designedpitch amount between the notching mold (or the blanking mold) and theforming mold, thereby positioning the notched material in the vicinityof the forming mold. In this case, the vicinity of the forming mold mayrefer to a space in which forming is performed between the forming uppermold and the forming lower mold.

According to an aspect of the present disclosure, the method may furtherinclude, after the forming step, one or more steps of a piercing step ofremoving an unnecessary portion such as a hole portion from the formedmaterial and a flanging step of forming a flange portion in the formedmaterial. That is, one or more steps of the piercing step and theflanging step may be performed between the forming step described aboveand a trimming step to be described below.

That is, according to an aspect of the present disclosure, in thepiercing step, the formed material may be transferred and positioned inthe vicinity of the piercing mold, and then an unnecessary portion suchas a hole portion may be removed from the formed material using thepiercing mold (that is, by combining an upper mold and a lower mold ofthe piercing mold). In this case, the vicinity of the piercing mold mayrefer to a space in which piercing is performed between the piercingupper mold and the piercing lower mold.

In addition, in the flanging step, the formed material (alternatively,as a case of including both the piercing step and the flanging step,when the piercing step and the flanging step are sequentially performed,the formed material refers to a “pierced material”) may be transferredand positioned in the vicinity of the flanging mold, and then the formedmaterial may be processed to form a flange portion in the piercedmaterial using the flanging mold (that is, by combining the upper moldand the lower mold of the flanging mold). In this case, the vicinity ofthe flanging mold may refer to a space in which flanging is performedbetween the flanging upper mold and the flanging lower mold.

Meanwhile, according to an aspect of the present disclosure, in a casein which both the piercing step and the flanging step described aboveare included, the order of the piercing step and the flanging step isnot particularly limited. That is, the piercing step and the flangingstep only need to be performed between the forming step and the trimmingstep, and piercing and flanging may be formed sequentially, orpiercing-flanging may be performed.

According to an aspect of the present disclosure, when the material istransferred from the vicinity of the forming mold to the vicinity of thepiercing mold (or the flanging mold), the formed material may betransferred by a designed pitch amount to correspond to the designedpitch amount between the forming mold and the piercing mold (or theflanging mold), thereby positioning the formed material in the vicinityof the piercing mold (or the flanging mold). Meanwhile, as a caseincluding both the piercing step and the flanging step, even when thematerial is transferred from the piercing mold to the flanging mold, thematerial may be transferred in the same manner as described above.

According to an aspect of the present disclosure, one or more steps ofthe piercing step and the flanging step may be performed in a singleprocess, and may be performed in a multistage process by being dividedinto two or more steps. Alternatively, one or more steps of the piercingstep and the flanging step may be omitted, if necessary.

In addition, according to an aspect of the present disclosure, themethod may include the trimming step of removing an unnecessary outeredge portion of the material from a final product shape using thetrimming mold. A material having a desired final product shape may beobtained by such a trimming step.

According to an aspect of the present disclosure, in the trimming step,the formed material (here, in a case of further including one or moresteps of the piercing step and the flanging step described above, theformed material refers to a material subjected to one or more steps ofthe piercing step and the flanging step) may be transferred andpositioned in the vicinity of the trimming mold, and then an unnecessaryouter edge portion is removed from the material using the trimming mold,thereby manufacturing a material having a final product shape. In thiscase, the trimming step may be performed on the material transferred tothe vicinity of the trimming mold (a space in which trimming isperformed between the trimming upper mold and the trimming lower mold)by combining the upper mold and the lower mold of the trimming mold.

According to an aspect of the present disclosure, when the material istransferred from the forming step to the trimming step, the material maybe transferred from the mold in the pre-transfer step (previous step) tothe vicinity of the mold in the post-transfer step (subsequent step) bya designed pitch amount to correspond to a designed pitch amount for twomolds adjacent to each other.

According to an aspect of the present disclosure, the trimming step maybe performed in a single process, or may be performed in a multistageprocess by being divided into two or more steps.

According to an aspect of the present disclosure, one or more of one ormore steps of the notching step and the blanking step; the forming step;and the trimming step (in a case of further including one or more stepsof the piercing step and the flanging step described above, includingthe step described above) may be performed in a multistage process bybeing divided into two or more steps. As such, each process is performedin a multistage process, such that formability may be further improvedin comparison to a case of performing each process in a single process,and a thickness reduction rate in each process may be reduced, which mayfurther improve the effect of preventing occurrence of cracks in thefinal product.

According to an aspect of the present disclosure, the method may furtherinclude a step of cooling a product having the trimmed final shape toroom temperature.

In addition, according to an aspect of the present disclosure, one ormore of one or more steps of the notching step and the blanking step;the forming step; and the trimming step (in a case of further includingone or more steps of the piercing step and the flanging step describedabove, including the step described above) may further include a step ofcooling the mold. For example, in a case in which the forming step isperformed in a multistage process by being divided into two or moresteps, the secondary forming step may further include a step of coolingthe secondary forming mold.

Meanwhile, in a common hot press forming method, forming is completed inone process, and the material is rapidly cooled to secure strength whilethe material is held in the mold for a predetermined time immediatelyafter the forming. On the other hand, in the technique of the presentdisclosure, cooling is performed through several processes such asnotching, blanking, forming, piercing, flanging, and/or trimming.

Therefore, according to an aspect of the present disclosure, as a resultof an earnest examination on the availability of the physical propertiesof the material through the multistage process described above, thepresent inventors have further found that the cooling rate of thematerial that may determine the availability of the physical propertiesof the material may be influenced by SPM, which is a press rate, thenumber of processes until the final product is taken out, the holdingtime to stay in the vicinity of the bottom dead center to close theupper mold and the lower mold in one process, the relational expressionof the slide stroke, and the like.

That is, according to an aspect of the present disclosure, themultistage process may be performed with the number of processes equalto or greater than a minimum number of processes calculated by thefollowing Relational Expression 1. Through this, a product havingdesired physical properties may be obtained.

N=ROUNDUP{T/[(60/SPM)×(f/100)]}  [Relational Expression 1]

(In Relational Expression 1, N represents a minimum required number ofprocesses from the forming step except for the notching step and theblanking step,

SPM represents the number of strokes per minute (SPM) of the press,

f represents a percentage (%) of a holding time in the vicinity of thebottom dead center in one stroke,

T represents a value calculated from the following Relational Expression2 when 0.8≤t≤1.5, and represents a value calculated from the followingRelational Expression 3 when 1.5≤t, and

ROUNDUP represents a value obtained by rounding up the number below thedecimal point of the calculated value within { }.)

T=t  [Relational Expression 2]

T=5×t−6  [Relational Expression 3]

(In Relational Expressions 2 and 3, t is a thickness of the material,and a unit thereof is mm.)

In addition, according to an aspect of the present disclosure, f maysatisfy the following Relational Expression 4 when 0.8≤t≤1.5 issatisfied, and may satisfy the following Relational Expression 5 when1.5 t is satisfied.

0.8×t+2.6≤f  [Relational Expression 4]

4.4×t−2.8≤f  [Relational Expression 5]

(In Relational Expressions 4 and 5, t is a thickness of the material,and a unit thereof is mm.)

According to an aspect of the present disclosure, as the thickness ofthe material is increased, cooling is not easily performed, and thus thecontact time with the mold needs to be longer. Therefore, as thethickness of the material is increased, a pressing motion having a largeholding percentage at the bottom dead center may be selected.

Alternatively, according to an aspect of the present disclosure, thecontact time with the mold may be approached by increasing the number ofprocesses, but as an air-cooling time is increased due to too manyprocesses in a state of little contact with the mold in one process, theroom for occurrence of other phases increases. Therefore, mostpreferably, it is required to increase the percentage of the holdingtime for the pressing slide to stay in the vicinity of the bottom deadcenter in one process. In addition, the contact time with the moldaffects not only the percentage of the holding time for the slide tostay in the vicinity of the bottom dead center, but also the duration ofone process, that is, the pressing motion rate (SPM).

Therefore, according to an aspect of the present disclosure, a finalproduct having desired strength may be obtained by performing amultistage process in which the number of processes is equal to orgreater than the minimum required number of processes calculated fromRelational Expression 1, and securing a minimum f value according to thethickness of the material represented in Relational Expressions 4 and 5.

According to an aspect of the present disclosure, in the notching stepand the blanking step, it is required to suppress cooling of thematerial, while in the multistage process after the notching step andthe blanking step (that is, from the forming step), it is required topromote cooling so as to secure a temperature below an Ms temperature inthe final product. Therefore, it is required to increase the contacttime with the mold for rapid cooling, and to this end, the followingmethod may be used.

In a common progressive method, the position of the strip material islifted in conjunction with lifting of the press upper mold, and thestrip material waits for transfer to the subsequent step. However, inthis case, the formed product (or the material) is cooled in contactwith the upper mold and the lower mold of the mold only while thepressing slide stays in the vicinity of the bottom dead center, and asthe pressing slide is lifted, the formed product is also lifted andseparated from the upper mold and the lower mold of the mold, and thusthe formed product is cooled only to a level of air-cooling.

Therefore, as a result of earnestly examining a preferred exemplaryembodiment of the multistage process by the present inventors, it isfound that since the strip material is not transferred during lifting ofthe slide upper plate (pressing slide), as illustrated in FIGS. 15through 17 , the pressing slide may be lifted from the pressing bottomdead center to a predetermined height, and the material (or the formedproduct) may stay in the vicinity of the pressing bottom dead centerbefore the transfer step of the strip material. It was further foundthat faster cooling could be secured in a case of applying a method ofallowing the material to stay in the vicinity of the pressing bottomdead center during lifting of the pressing slide and lifting thematerial immediately before the point of the transfer step of the stripby utilizing the position information of the pressing slide.

FIGS. 15 through 17 illustrate the pressing motion of the exemplaryprocessing apparatus 41 of the present disclosure. That is, according toan aspect of the present disclosure, as illustrated in FIG. 15 , whenthe material is put into the processing apparatus 41, a pressing slide 6a is lowered to a pressing bottom dead center 120 b to process thematerial. Next, as illustrated in FIG. 16 , the material stays in thevicinity of the pressing bottom dead center 120 b during lifting of thepressing slide 6 a. Finally, as illustrated in FIG. 17 , when thepressing slide is lifted and reaches a point 120 a that does notinterfere with the transfer of the material, a method of lifting thematerial positioned in the vicinity of the pressing bottom dead center120 b may be applied by utilizing the position information of thepressing slide.

According to an aspect of the present disclosure, the method of liftingthe material described above is illustrated in FIG. 17 . Specifically,when the pressing slide 6 a is lifted and then reaches the point 120 athat does not interfere with the transfer of the material, the materialmay be lifted from the vicinity of the pressing bottom dead center tothe transfer point 120 a by a material position control unit 600 in theform of a cylinder that controls the material to be separated from lowermolds 61 b, 62 b, 63 b, and 64 b of a mold used in each step.

In this case, according to an aspect of the present disclosure, thematerial position control unit 600 may be provided on the surface onwhich the mold on the press lower plate (that is, the press bolster) isprovided. More specifically, the material position control units 600 maybe provided at both ends of one mold (that is, anyone of the lower molds61 b, 62 b, 63 b, and 64 b) in the transfer direction of the material.Alternatively, as illustrated in FIG. 17 , one material position controlunit 600 may be provided between two molds adjacent to each other.

In addition, according to an aspect of the present disclosure, in a casein which the forming step is performed in a multistage process with twoor more steps, in order to prevent occurrence of cracks due to excessivecooling of a portion to be formed, the forming step may further includea step of heating one or more forming molds of additional forming moldsother than the primary forming mold used in the primary forming step.

Alternatively, according to an aspect of the present disclosure, one ormore steps of the piercing step and the flanging step, and/or thetrimming step may further include a step of heating one or more molds ineach step in order to prevent occurrence of cracks due to excessivecooling of the processed portion.

A three-dimensional structure of the material after passing through themultistage process according to the exemplary progressive method of thepresent disclosure is illustrated in FIG. 12A. Specifically, a notchedmaterial 210 after passing through the notching step as a first processhas a shape connected to the strip by a web portion 300. Subsequently, aformed material 220 after passing through the forming step as a secondprocess forms a three-dimensional structure. In this case, a side viewof the three-dimensional structure is illustrated in FIG. 12B. Inaddition, a shape of a pierced material 230 after passing through thepiercing step as a third process is illustrated, and an unnecessary holeportion is removed from the material. In addition, a trimmed material240 after passing through the trimming step as a fourth process isillustrated, and an unnecessary outer edge portion 400 is removed byperforming the trimming step. By performing all the processes describedabove, a final product (250 in FIG. 11 ) taken out from the processingapparatus may be obtained.

In addition, although not particularly illustrated in the drawing, in acase of including the blanking step of obtaining a blank materialseparated from a strip according to a transfer method to be describedbelow, the material may be processed in the same manner, except that ablank separated from the strip is formed so as not to include the stripweb portion as illustrated in FIG. 12 , and then a subsequent process isperformed. In this case, the shape of the material may be formed in thesame manner as in the case of using a common method in the art.

Meanwhile, according to an aspect of the present disclosure, in the caseof the progressive method, the material is transferred by forming amaterial guide portion in the web portion 300 and operating the materialguide portion. The strip web portion 300 is a portion that is notactually formed and is removed from the final product shape in thetrimming step. Therefore, the web portion 300 may be heated at a lowtemperature in the processing process or may be cooled faster than theportion of the material to be processed in the notching step so as topromote smooth transfer of the material.

That is, according to an aspect of the present disclosure, in thenotching step, the cooling of the web portion of the material may beperformed faster than the cooling of the formed portion of the material.

Alternatively, according to an aspect of the present disclosure, in thenotching step, the web portion of the material may be controlled to atemperature lower than that of the formed portion of the material of theproduct. As such, in order to control the web potion of the material toa lower temperature than other portions, in the notching mold, only apart of the mold corresponding to the web portion of the material may becontrolled to a temperature lower than those of other portions.

In addition, in the hot press forming method for a multistage processaccording to an aspect of the present disclosure, it is required toachieve uniform heating by rapidly heating the strip material and tominimize a decrease in temperature by minimizing the contact time withthe mold in one or more steps of the notching step and the blanking stepfor a high-temperature strip material. On the other hand, in the formingstep, the piercing step, the flanging step, the trimming step, and thelike (here, one or more steps of the piercing step and the flanging stepmay be omitted) after the one or more steps of the notching step and theblanking step, it is required to secure fast cooling by maximizing thecontact time of the material with the mold. As such, in a case in whichthe hot press forming method is performed to the final step by theprogressive method, one or more steps of the notching step and theblanking step, the forming step, the piercing step, the flanging step,and the trimming step all proceed to the same transfer point, and thusconflicting requirements make it difficult to achieve different goals.

Therefore, as a result of an earnest examination, the present inventorshave further found that there are many advantages to using theprogressive method in the heating step, the notching step, and theblanking step in order to satisfy the conflicting requirements describedabove in each step, and the contact with the mold may be controlled fora longer period of time using the transfer method in the forming step,the piercing step, the flanging step, and the trimming step.

According to an aspect of the present disclosure, it is confirmed thatphysical properties are secured more advantageously by applying acomplex method in which the heating step, the notching step, and theblanking step are performed by the progressive method and the formingstep, the piercing step, the flanging step, and the trimming step areperformed by the transfer method.

That is, according to an aspect of the present disclosure, the hot pressforming method for a multistage process may include the blanking step ofobtaining a blank material separated from the strip by cutting a part ofthe heated strip material put into the processing apparatus, and it ispossible to apply a transfer method using a tongs-shaped transfer unitwhen the material is transferred from the mold in the pre-transfer step(previous step) to the vicinity of the mold in the post-transfer step(subsequent step) based on two molds adjacent to each other in thetransfer direction of the material in one press.

For example, when the material is transferred from the blanking step tothe forming step, and when the material is transferred from the formingstep to the subsequent step (the piercing step, the flanging step, orthe trimming step), the material positioned in the vicinity of the moldin the previous step may be transferred to the vicinity of the mold inthe subsequent step using the tongs-shaped transfer unit.

[Apparatus for Manufacturing Hot Press Formed Member for MultistageProcess]

Another aspect of the present disclosure provides an apparatus formanufacturing a hot press formed member for a multistage process, theapparatus including:

a supply unit for continuously supplying a strip material;

a heating unit for heating the strip material;

a processing unit including a processing apparatus in which a pluralityof molds including one or more molds of a notching mold and a blankingmold, a forming mold, and a trimming mold are mounted on one press; and

a transfer unit for transferring the strip material heated in theheating unit to the processing unit.

Meanwhile, the description of the method for manufacturing a hot pressformed member for a multistage process described above may be similarlyapplicable to the apparatus for manufacturing a hot press formed memberfor a multistage process.

Specifically, an exemplary structure of the apparatus for manufacturinga hot press formed member for a multistage process of the presentdisclosure is illustrated in FIG. 1 , and an apparatus 100 formanufacturing a hot press formed member for a multistage processincludes a supply unit 1, a heating unit 2, a transfer unit 3, and aprocessing unit 4.

According to an aspect of the present disclosure, the supply unit 1continuously supplies a strip material 200. In this case, the supplyunit 1 may continuously supply a material provided in the form of a coilto the heating unit 2 in the form of a strip material having a uniformwidth by an uncoiler.

According to an aspect of the present disclosure, the heating unit 2 mayheat the strip material supplied from the supply unit described aboveusing a heating device 21. In this case, the rapid heating methoddescribed above may be applied, and for example, various heating devicessuch as an induction heating device, a resistance heating device, and aninfrared heating device may be used.

According to an aspect of the present disclosure, the transfer unit 3may transfer the strip material 200 heated in the heating unit 2 to theprocessing unit 4. In this case, the transfer of the strip material 200may be performed in a thermal insulation chamber 31. Meanwhile, the samedescription described above may apply to the thermal insulation chamber.

According to an aspect of the present disclosure, the processing unit 4may include a processing apparatus 41 in which one or more molds 61 aand 61 b of a notching mold and a blanking mold, forming molds 62 a and62 b, and trimming molds 64 a and 64 b are mounted on one press 6, 6 a,or 6 b. Meanwhile, in the forming method, as described above, one ormore molds of a piercing mold and a flanging mold may be further mountedbetween the forming mold and the trimming mold.

According to an aspect of the present disclosure, the one press 6 mayinclude a pressing slide corresponding to an upper plate 6 a of thepress and a press bolster corresponding to a lower plate 6 b of thepress. In this case, the lower plate 6 b of the press may be provided toface the upper plate 6 a of the press.

According to an aspect of the present disclosure, the processingapparatus 41 may further include additional temperature control unit 5 aand 5 b for the mold for controlling the temperature of each molddescribed above, and the temperature control unit for the mold may beprovided between the mold and the press. That is, the temperaturecontrol unit for the mold may be provided on the press in contact witheach mold.

According to an aspect of the present disclosure, in a case of using aheated mold as the notching mold and/or the blanking mold, theprocessing apparatus 41 may further include additional temperaturecontrol units 51 a and 51 b for the notching mold and/or the blankingmold for controlling the temperature of the mold. In this case, thetemperature control units 51 a and 51 b may be provided between themolds 61 a and 61 b and the presses 6 a and 6 b.

In addition, according to an aspect of the present disclosure, atemperature control unit for each mold may be additionally providedbetween each mold and press in the same manner as described above (forexample, temperature control units 52 a and 52 b for the forming molds,temperature control units 53 a and 53 b for the piercing molds or theflanging molds, and temperature control units 54 a and 54 b for thetrimming molds, see FIG. 1B).

According to an aspect of the present disclosure, as described above, asa structure protruding from a surface of the notching upper mold or theblanking upper mold in contact with the strip material, a push bar 11operated by a spring may be provided (see FIG. 14 ). Meanwhile, the samedescription described above may apply to the push bar.

According to an aspect of the present disclosure, as described above, amaterial position control unit 600 in the form of a cylinder may beprovided on a surface on which the mold on the press lower plate (thatis, the press bolster) is provided. The same description described abovemay apply to the material position control unit 600.

In addition, according to an aspect of the present disclosure, theprocessing apparatus may include blanking molds as a plurality of molds,and may further include a tongs-shaped transfer unit for transferringthe material from the mold in a previous step to the vicinity of themold in a subsequent step based on two molds adjacent to each other in atransfer direction of the material. In this case, the tongs-shapedtransfer unit is an example of applying a transfer method, and the samedescription described above may be applied.

In addition, the same contents as those of the method for manufacturinga hot press formed member for a multistage process described above maybe applied to the apparatus for manufacturing a hot press formed memberfor a multistage process according to an aspect of the presentdisclosure. Therefore, as an example, a processing apparatus that mayperform a multistage process in which the number of processes is equalto or greater than the minimum required number of processes calculatedfrom Relational Expression 1 may be used. For example, in the processingapparatus described above, the number of the plurality of molds mountedon one press may be equal to or greater than the minimum number ofprocesses calculated from Relational Expression 1 described above.Alternatively, a processing apparatus satisfying Relational Expressions4 and 5 described above may be used by utilizing the information on thethickness (corresponding to t in Relational Expressions 4 and 5) of thematerial put into the processing apparatus.

[Hot Press Formed Member]

Still another aspect of the present disclosure provides

a hot press formed member manufactured by the method for manufacturing ahot press formed member for a multistage process,

wherein the hot press formed member has an under-cut shape, and atensile strength of the hot press formed member is 1,300 MPa or more.

According to an aspect of the present disclosure, the under-cut shapehas the same meaning as the term used in the art. Accordingly, theunder-cut shape may include various shapes commonly interpreted in theart. For example, since a convex or concave portion is included in aside portion or a burring portion or the like is formed in the sideportion, a formed shape of a hot press formed member may include a shapethat may not form a formed product or may not be taken out by only avertical movement of the mold.

As a non-limiting example of the present disclosure, various shapes ofthe hot press formed member are illustrated in FIG. 18 . The hot pressformed member according to an aspect of the present disclosure mayinclude a flat portion (A) having a first plane direction (S) and a sideportion (B) having a second plane direction (P) different from the firstplane direction. In this case, the hot press formed member may have oneor more flat portions and/or one or more side portions. Meanwhile, inthe hot press formed member, any one of the flat portions (A) and anyone of the side portions (B) may be connected to each other, and sincethe flat portion (A) and the side portion (B) described above have planedirections different from each other, the flat portion (A) and the sideportion (B) are connected to each other to provide a bent portion.

For example, in a case in which the hot press formed member has a hatshape as illustrated in FIG. 18 , a first side portion and a second sideportion may be connected to each of both ends of one flat portion (thatis, the first side portion may be connected to one end of both ends ofone flat portion, and the second side portion may be connected to theother end). In addition, a first flat portion may be additionallyconnected to the end of the first side portion other than the endconnected to the flat portion, and a second flat portion may beadditionally connected to the end of the second side portion other thanthe end connected to the flat portion (however, the hot press formedmember may further include, but is not limited to, additional flatportions and/or side portions). Therefore, as a non-limiting example ofthe present disclosure, the hot press formed member may be composed ofthree flat portions (A) and two side portions (B) as illustrated in FIG.18 .

Meanwhile, as a non-limiting example of the present disclosure, asillustrated in the side view of the formed member of FIG. 18B, theunder-cut shape may include a portion where an angle (D) of a narrowside formed by the first plane direction (S) of one flat portion (A) andthe second plane direction (P) of one side portion (B) is within 90°.

Alternatively, as another non-limiting example of the presentdisclosure, as illustrated in the perspective view of the formed memberof FIG. 18A, the under-cut shape may include an additional formedportion (C) (for example, including a burring portion or the like) inone or more of any one of the flat portions (A) and any one of the sideportions (B). In this case, the formed member may include a bent portionformed by connecting any one of the flat portions (A) and any one of theside portions (B). Accordingly, the formed portion (C) may be providedon a plane of the side portion in the second plane direction.

According to an aspect of the present disclosure, in a case in which theforming step described above is performed in a multistage process withtwo or more steps, the under-cut shape may be formed by a manufacturingmethod in which a forming direction of the material in one forming stepand a forming direction of the material in the other forming step aredifferent from each other. Specifically, the under-cut shape may meanthat two or more portions having forming directions different from eachother are included, and in this case, the same description describedabove may apply to the forming direction.

For example, as described above, in a case in which a burring portion isformed in the primary forming step and then bending processing isperformed in the secondary forming step, since the forming directions inthe primary forming step and the secondary forming step are differentfrom each other, it is possible to form an under-cut shape that may notbe formed when the forming step is performed in a single process.

Alternatively, as another example, the material may be processed in theprimary forming step to include a flat portion and a side portion, andthen pressure may be applied to the material in a directionperpendicular to a plane of the side portion in the secondary formingstep, thereby forming an under-cut shape in which an angle formed by theflat portion and the side portion is within 90°.

Meanwhile, according to an aspect of the present disclosure, the hotpress formed member may have an overall tensile strength of 1,300 MPa ormore even when it has an under-cut shape, and an upper limit of thetensile strength may not be particularly limited because the higher thestrength characteristic, the better the characteristic. However, as anon-limiting example, the upper limit of the tensile strength may be2,000 MPa.

MODE FOR INVENTION Examples

Hereinafter, the present disclosure will be described in more detailwith reference to Examples. However, it should be noted that thefollowing Examples are provided to describe the present disclosure byway of illustration, but are not intended to limit the scope of thepresent disclosure. This is because the scope of the present disclosureis determined by contents disclosed in the claims and contentsreasonably inferred therefrom.

Experimental Example 1

A strip material having a uniform width and a blank material having anon-uniform width were prepared, and resistance heating was performedfor each material at a heating rate of 50° C./s. At this time, an alloyplated steel sheet having an Fe content of 30 wt % in a surface portionof a plating layer (within 2 μm from the surface of the plating layer)was used as a material, the alloy plated steel sheet obtained byalloying a plated steel sheet formed by immersing, in a plating bathAl-Si9%-Fe3%, a base steel sheet having a composition containing, by wt%, 0.22% of C, 0.3% of Si, 1.2% of Mn, 0.2% of Cr, 0.03% of Al, 0.01% ofP, 0.001% of S, 0.003% of N, 0.003% of B, and a balance of Fe and otherunavoidable impurities.

A temperature distribution result at the time of the resistance heatingof the strip material having a uniform width is illustrated in FIG. 2Aas Example 1, and a temperature distribution result at the time of theresistance heating of the strip material having a non-uniform width isillustrated in FIG. 2B as Comparative Example 1.

As can be seen by comparing the temperature distribution results of FIG.2 , in Comparative Example 1, as illustrated in FIG. 2B, a largertemperature difference occurred in a narrow region and a wide region ofthe material in comparison to FIG. 2A. Therefore, in the case of Example1 using the strip material having a uniform width in FIG. 2A, it wasconfirmed that more uniform heating was easily performed by applyingrapid heating in comparison to Comparative Example 1 using the blankhaving a non-uniform width.

Experimental Example 2

The prepared strip material having a thickness of 1.4 mm was rapidlyheated to 920° C. at a heating rate of 50° C./s using a resistanceheating method with an effective length of a heating device having 2 mand a cycle time of 4 seconds. In this case, the same alloy plated steelsheet used in Experimental Example 1 was used as the strip material.Subsequently, the heated strip material was passed through a heatingchamber at 900° C. and then transferred to a processing apparatus. Thetransferred strip material was operated at 15 SPM using a pressingmotion by a crank motion method of FIG. 3A. The case in which nothingwas performed using a notching mold in a cold state having an initialtemperature of 25° C. was used as Example 2, and the case in whichnotching was performed using a notching mold in a heated state having aninitial temperature of 500° C. was used as Example 3. A graph of thetemperature change of the material over time in the notching step ofeach of Examples 2 and 3 is illustrated in FIG. 4 .

In the case of Example 2, cooling of the material was significantlyquickly performed from when a notching upper mold was lowered and cameinto contact with the material by using the notching mold in a coldstate. On the other hand, in the case of Example 3, excessive coolingwas suppressed when the notching upper mold was lowered and then cameinto contact with the material using the heated notching mold having aninitial mold temperature of 50° C. or higher. That is, in Example 3,excessive cooling was suppressed in comparison to Example 2, such thatafter completion of the notching step, the temperature of the materialwas controlled to 700° C. or higher. Therefore, it was confirmed thatthe elongation of the material subjected to the notching step of 50% ormore was secured, which was a level sufficient to perform the formingprocess in a subsequent process, and thus formability in the subsequentprocess was more excellent.

Experimental Example 3

A strip material was processed in the same manner as that ofExperimental Example 2 described above, except that a strip materialhaving a thickness of 1 mm was used and the initial temperature of thenotching mold was changed as shown in Table 1. At this time, a surfacetemperature of the material was measured based on the point at whichcontact of the notching mold with the material was finished in thenotching step. The result thereof is shown in Table 1.

Meanwhile, in order to confirm the effect of availability of additionalforming according to the temperature of the notching mold, an elongationof the material obtained after the notching step was measured toevaluate availability of additional forming based on the followingcriteria.

x: Elongation of less than 50%

∘: Elongation of 50% or more

At this time, in Comparative Example 2, when the blank itself, not thestrip material, was rapidly heated, a surface temperature of thematerial was measured. The result thereof is shown in Table 1.Availability of additional forming was measured in the same manner asdescribed above.

In addition, the uniformity of the temperature of the material wasmeasured in the same manner as that of Experimental Example 1 describedabove, and evaluation was performed according to the following criteria.

x: Case in which the temperature difference generated in the materialwas 50° C. or higher

∘: Case in which the temperature difference generated in the materialwas lower than 50° C.

TABLE 1 Initial Avail- temperature Surface Uniformity ability of ofnotching temperature of tem- additional No. mold of material peratureforming Comparative 500° C. 550 to 720° C. x x Example 2 Example 4  25°C. 608° C. ∘ x Example 5  50° C. 612° C. ∘ x Example 6 200° C. 656° C. ∘x Example 7 300° C. 682° C. ∘ x Example 8 400° C. 712° C. ∘ ∘ Example 9500° C. 742° C. ∘ ∘ Example 10 600° C. 772° C. ∘ ∘ Example 11 700° C.801° C. ∘ ∘

As shown in Table 1, in Comparative Example 2, a high surfacetemperature of the material was secured by heating the blank itself, butdue to the non-uniform temperature distribution caused by the shape ofthe blank material having a non-uniform width, after the blanking step,a portion of the material having a temperature that was too low wasformed, resulting in unavailability of additional forming.

On the other hand, in the cases of Examples 4 to 11 in which the stripmaterial was rapidly heated, it was confirmed that the temperatureuniformity of the material was superior to that in Comparative Example2.

Meanwhile, in Examples 8 to 11 in which the initial temperature of thenotching mold was 400° C. or higher, since the temperature of thematerial after notching was higher than those in Comparative Examples 4to 7 in which the initial temperature of the notching mold was lowerthan 400° C., a desired level of an elongation was secured, and thusadditional forming was possible.

Experimental Example 4

A strip material was notched in the same manner as that of ExperimentalExample 2, except that the operation was performed using a pressingmotion by a link method of FIG. 3B, a push bar was provided on a surfaceof the notching upper mold in contact with the strip material so thatthe notching upper mold and the notching lower mold were in contact withthe strip material only in the cutting process in the notching step, andthe push bar was controlled to be operated by a spring. At this time,the case in which the initial temperature of the notching mold was 25°C. was used as Example 12, and the case in which the initial temperatureof the notching mold was 500° C. was used as Example 13. A graph of thetemperature change of the material over time in the notching step isillustrated in FIG. 5 .

Meanwhile, it was conformed from the comparison of FIGS. 4 and 5 that inExamples 12 and 13, the cooling of the material was significantlyreduced as the contact time with the notching upper mold was decreasedin comparison to Examples 2 and 3. As such, when the press was loweredand lifted, the notching upper mold was lowered and lifted withoutcontact with the strip material, and the notching upper and lower moldswere brought into contact with the strip material only in the cuttingprocess, such that the surface temperature of the material after thenotching step was controlled to be higher. It was confirmed from thisthat a high elongation of 50% or more of the material was secured, andmore excellent formability was confirmed in the subsequent process.

Experimental Example 5

A steel sheet having the same composition as used in ExperimentalExample 1 described above as the prepared strip material having athickness of 1.4 mm was rapidly heated to 920° C. at a heating rate of50° C./s using a resistance heating method. Subsequently, the heatedstrip material was transferred to a processing apparatus in a thermalinsulation chamber at 900° C. The transferred strip material wasoperated at 15 SPM using the motion method shown in Table 2 and thepressing motion satisfying a holding percentage in the vicinity of thebottom dead center in one stroke, and a notching step, a forming step, apiercing step, and a trimming step of the HAT-shaped product wereperformed. At this time, in the notching step, the notching mold havingan initial temperature satisfying Table 2 was used, and the notchingupper mold and the notching lower mold were controlled to be in contactwith the strip material only in the cutting process using a push bar.

The formability, whether or not a martensite (Ms) phase was secured, andthe strength characteristic of Examples 14 to 16 satisfying theexperimental conditions of Table 2 were evaluated. The results are shownin Table 2.

In this case, the formability was evaluated on the same criteria asthose of Experimental Example 3 described above. As for the strengthcharacteristic of the product, a fraction of 99% or more of themartensite phase in the final product was secured based on the formingand cooling analysis method considering the phase transformation of thematerial, and thus the tensile strength was 1,300 MPa or more wasindicated as “∘”, and the case in which a fraction of the martensitephase secured in the final product was 90% or less was indicated as “x”.

TABLE 2 Holding Initial percentage in temper- Motion vicinity ofStrength ature of method of bottom dead charac- notching pressing centerin one Form- teristic No. mold motion stroke ability of productComparative — FIG. 3(A) 3.2% x x Example 3 Example 14  25° C. FIG. 3(A)3.2% ∘ x Example 15 500° C. FIG. 3(A) 3.2% ∘ x Example 16 500° C. FIG.3(B)  10% ∘ ∘

The temperature changes of the material over time during the multistageprocess of Examples 14 to 16 shown in Table 2 are illustrated in FIGS. 6through 8 , respectively. In this case, as for the temperature change ofthe material, a temperature change on the surface of the final productis illustrated based on the point that is the circled portion in each ofFIGS. 6 through 8 .

In the case of Comparative Example 3 in which the multistage process wasperformed in the same conditions as those in Examples 14 to 16 exceptfor applying the method of rapidly heating the blank material accordingto the related art, due to the non-uniform temperature distributioncaused by the shape of the blank material having a non-uniform width, aportion of the material having a temperature that was too low wasformed, resulting in unavailability of additional forming.

Meanwhile, in the case of Example 14, it was confirmed that theformability was superior to that in Comparative Example 3. However, asillustrated in FIG. 6 , the cooling rate was low because the contacttime with the mold was rather short, and thus 99% or more of sufficientmartensite was not secured.

On the other hand, in the case of Example 15, as illustrated in FIG. 7 ,a decrease in temperature in a section of 0 to 4 seconds correspondingto the notching step was rather reduced, and the formability wassuperior to that in Example 14. However, the contact time with the moldand the formed product was rather short, and thus 99% or more ofsufficient martensite was not secured.

In addition, in the case of Example 16, as illustrated in FIG. 8 , sincea decrease in temperature in the notching step was low, the formabilitywas excellent and the strength characteristic of the product was alsomore excellent.

Specifically, in Example 16, since a percentage of a holding time forthe upper mold and the lower mold of the mold to stay at the pressingbottom dead center in a closed state was higher than those in Examples14 and 15, a fast cooling rate of the material in the whole process wassecured. Therefore, after the multistage process was performed for 16seconds in total, the temperature of the material was secured below theMs temperature of 400° C., and thus a critical cooling rate was secured,such that a martensite phase was sufficiently secured after themultistage process. As a result, a product having a desiredpredetermined strength characteristic was obtained.

Experimental Example 6

When the notched material manufactured by the method of ExperimentalExample 2 was manufactured, the case in which the notching step wasperformed in a single process as illustrated in FIG. 9A was used asExample 17. A thickness reduction rate at this time is illustrated inFIG. 9B. Similarly, the case in which the notching step was performed bybeing divided into a two-step process as illustrated in FIG. 10A wasused as Example 18. A thickness reduction rate at this time isillustrated in FIG. 10B.

It was confirmed that in the case of Example 18, since the notching stepwas performed by being divided into two steps, a thickness reductionrate in the final product was further reduced, and therefore, the effectof preventing occurrence of cracks was more excellent, in comparison toExample 17 in which the notching step was performed in a single process.

Experimental Example 7

The evaluation results of the pressing motion rate (SPM) and thepercentage of the holding time for the slide to stay in the vicinity ofthe bottom dead center in one process, whether or not the material takenout is secured below the Ms temperature according to the change in theminimum required number of processes from the forming step excludingnotching, whether or not a stable martensite phase is secured, and thephysical properties of the product depending on several thicknesses ofthe materials having the same composition as in Experimental Example 2described above are shown in Table 3.

In this case, the formability was evaluated based on the same criteriaas those in Experimental Example 2 described above, as for whether ornot a stable martensite phase was secured, based on the same criteria asthose in Experimental Example 5 described above, the case in which 99%or more of the martensite phase was secured was indicated as “OK”, andthe case in which 90% or less of the martensite phase was secured wasindicated as “NOK”. In addition, the evaluation of the product strengthcorresponds to “OK” described above. The case in which the tensilestrength was 1,300 MPa or more was indicated as “o”, and the other caseswere indicated as “x”.

TABLE 3 Whether or not to reach below Ms Whether or temperature notstable Material when Evaluation martensite Evaluation Experimentalthickness Number of removing of phase is of product Example [mm] f SPMprocesses product formability secured strength Comparative 1 12.5 15 3OK x NOK x Example 4 Example 19 1 5 15 5 OK ∘ NOK x Example 20 1 12.5 153 OK ∘ OK ∘ Example 21 1 5 10 3 OK ∘ NOK x Example 22 1 7.5 10 3 OK ∘NOK x Example 23 1 7.5 15 4 OK ∘ NOK x Example 24 1 9 15 4 OK ∘ OK ∘Example 25 1 9 15 2 NOK ∘ There is x room for formation of phases otherthan martensite phase at time of air-cooling after removing productExample 26 1.5 5 15 7 OK ∘ NOK x Example 27 1.5 12.5 15 4 OK ∘ OK ∘Example 28 1.5 12.5 15 2 NOK ∘ There is x room for formation of phasesother than martensite phase at time of air-cooling after removingproduct Example 29 1.5 7.5 15 5 OK ∘ NOK x Example 30 1.5 10 15 4 OK ∘OK ∘ Example 31 1.5 12.5 30 6 OK ∘ OK ∘ Example 32 2 5 15 8 OK ∘ NOK xExample 33 2 12.5 15 7 OK ∘ NOK x Example 34 2 25 15 3 NOK ∘ There is xroom for formation of phases other than martensite phase at time ofair-cooling after removing product Example 35 2 25 15 4 OK ∘ OK ∘Example 36 2 17.5 10 4 OK ∘ OK ∘

As shown in Table 3, in the case of Comparative Example 4 in which amethod of rapidly heating the blank material according to the relatedart was applied, despite the condition that the physical properties weresecured as in Example 20 due to the temperature non-uniformity of thematerial caused in the heating step, the formability was not excellentdue to the temperature non-uniformity of the material caused in theheating step, and the physical properties of the final product were notsecured.

Meanwhile, in Examples 19, 21 to 23, 26, 29, 32, and 33, the formabilitywas superior to that in Comparative Example 4, but the physicalproperties were not secured because other phases were formed due to theminimum f value that was smaller than the minimum f value required fromthe material thickness according to Relational Expressions 4 and 5described above. In addition, in Examples 25, 28, and 34, the minimum fvalue satisfied the minimum f value required from the material thicknessaccording to Relational Expressions 4 and 5 described above, but theminimum number of processes was smaller than the minimum number ofprocesses (N) calculated from Relational Expression 1. For this reason,the temperature of the material did not reach a temperature below the Mstemperature at the time of tanking out the product, and there was roomfor formation of phases other than martensite in the air-cooling processafter removing the product under the condition of having a temperaturebelow the Ms temperature. Therefore, when Relational Expressions 4 and 5described above were not satisfied or Relational Expression 1 was notsatisfied, it was required to solve the problem by removing the productafter performing more processes than the minimum required process (N) bya method such as adding a cooling process.

On the other hand, Examples 20, 24, 27, 30, 31, 35, and 36 are cases inwhich the minimum f value required from the material thickness accordingto Relational Expressions 4 and described above was satisfied, and themultistage process is performed with more than the minimum number ofprocesses (N) calculated from Relational Expression 1 described above.Accordingly, at the time of removing the product in the multistageprocess, the temperature below the Ms temperature was secured, andphases other than martensite were not formed, such that a productsecuring a sufficient martensite phase was obtained, and the physicalproperties of the final product were secured.

Experimental Example 8

A blanking step was performed in the same manner as that of ExperimentalExample 2 described above, except that in Examples 37 and 38, a blankingstep of preparing a blank material separated from the strip was includedinstead of the notching step, and then forming, piercing, and trimmingsteps were performed. At this time, when transfer was performed betweentwo or more molds adjacent to each other in the transfer direction ofthe material, a tongs-shaped transfer unit was used.

In the case of Example 37, the cooling of the material was performedsignificantly quickly because the blanking mold in a cold state havingan initial temperature of 25° C. was used, whereas in Example 38 inwhich the initial temperature was 500° C., when the blanking upper moldwas lowered and came into contact with the material, excessive coolingwas suppressed by using a heated blanking mold.

That is, it was confirmed that in Example 38, since excessive coolingwas suppressed, the temperature of the blank material after completionof the blanking step was 700° C. or higher, and a material having anelongation of 50% or more was secured, such that the formability in asubsequent process after the blanking step was more excellent incomparison to Example 37.

DESCRIPTION OF REFERENCE NUMERALS

-   -   100: Apparatus for manufacturing hot press formed member for        multistage process    -   1: Supply Unit    -   10: Material Provided In Form Of Coil    -   11: Push Bar    -   2: Heating Unit    -   21: Heating Device For Strip Material    -   3: Transfer Unit    -   31: Thermal Insulation Chamber    -   4: Processing Unit    -   41: Processing Apparatus    -   5 a, 5 b: Temperature Control Unit For Mold    -   50: Designed Pitch Amount    -   X: Transfer Direction Of Strip Material    -   51 a, 51 b: Temperature Control Unit For Notching Mold Or        Blanking Mold    -   52 a, 52 b: Temperature Control Unit For Forming Mold    -   53 a, 53 b: Temperature Control Unit For Piercing Mold Or        Flanging Mold    -   54 a, 54 b: Temperature Control Unit For Trimming Mold    -   6: Press    -   6 a: Press Upper Plate (pressing Slide)    -   6 b: Press Lower Plate (Press Bolster)    -   61 a, 61 b: Notching Mold Or Blanking Mold    -   62 a, 62 b: Forming Mold    -   63 a, 63 b: Piercing Mold Or Flanging Mold    -   64 a, 64 b: Trimming Mold    -   110 a: Transfer Height Level Of Strip Material    -   110 b: Vicinity Of Bottom Dead Center Of pressing Slide    -   120 a: Point That Does Not Interfere With Transfer Of Material    -   120 b: Vicinity Of pressing Bottom Dead Center    -   200: Material    -   210: Notched Material    -   220: Formed Material    -   230: Pierced Material    -   240: Trimmed Material    -   250: Final Product Taken Out    -   300: Web Portion    -   400: Unnecessary Outer Edge Portion    -   600: Material Position Control Unit    -   A: Flat Portion    -   B: Side Portion    -   C: Formed Portion (Burring Portion Or The Like)    -   S: First Plane Direction    -   P: Second Plane Direction    -   D: Angle Of Narrow Side Formed By First Plane Direction And        Second Plane Direction

1. A method for manufacturing a hot press formed member for a multistageprocess, the method comprising: a heating step of heating a stripmaterial; a transfer step of transferring the heated strip material to aprocessing apparatus in which a plurality of molds including one or moremolds of a notching mold and a blanking mold; a forming mold; and atrimming mold; are mounted on one press; one or more steps of a notchingstep of obtaining a notched material connected to the strip by a webportion by cutting a part of the material using the notching mold and ablanking step of obtaining a blank material separated from the strip bycutting a part of the material using the blanking mold; a forming stepof transferring the material subjected to one or more steps of thenotching step and the blanking step and positioning the material in thevicinity of the forming mold, and then forming the material using theforming mold; and a trimming step of removing an unnecessary outer edgeportion of the material from a final product shape using the trimmingmold.
 2. The method for manufacturing a hot press formed member for amultistage process of claim 1, wherein in the transfer step, the heatedstrip material is transferred in a thermal insulation chamber, and thethermal insulation chamber is maintained within a temperature range ofTs−200° C. or higher and Ts+50° C. or lower based on a surfacetemperature (Ts) of the strip material supplied to the thermalinsulation chamber.
 3. The method for manufacturing a hot press formedmember for a multistage process of claim 1, wherein in the one press,the molds are arranged to be spaced apart from each other by a designedpitch amount between two molds adjacent to each other in a transferdirection of the material, and after each stroke is performed accordingto a pressing motion, the material is transferred by the designed pitchamount in the transfer direction of the material.
 4. The method formanufacturing a hot press formed member for a multistage process ofclaim 1, wherein an initial temperature of the notching mold and aninitial temperature of the blanking mold are 400° C. or higher.
 5. Themethod for manufacturing a hot press formed member for a multistageprocess of claim 1, wherein one or more of one or more steps of thenotching step and the blanking step; the forming step; and the trimmingstep are performed in a multistage process by being divided into two ormore steps.
 6. The method for manufacturing a hot press formed memberfor a multistage process of claim 1, wherein in the notching step andthe blanking step, the strip material put into the processing apparatusis controlled so that the strip material is lowered to a pressing bottomdead center from a transfer height level of the strip material in anon-contact state with an upper mold of a mold, and then comes intocontact with an upper mold surface and a lower mold surface of the moldonly in a cutting process.
 7. The method for manufacturing a hot pressformed member for a multistage process of claim 1, wherein in a pressingmotion of the processing apparatus, a percentage of a holding time inthe vicinity of a bottom dead center in one stroke is 4 to 30%, and thepercentage of the holding time in the vicinity of the bottom dead centeris a percentage of a time for the press to stay to a point correspondingto 1 mm from a pressing bottom dead center in an upward direction. 8.The method for manufacturing a hot press formed member for a multistageprocess of claim 1, further comprising, between the forming step and thetrimming step, one or more steps of a piercing step of removing anunnecessary hole portion from a formed material and a flanging step offorming a flange portion in the formed material.
 9. The method formanufacturing a hot press formed member for a multistage process ofclaim 1, wherein the multistage process is performed with the number ofprocesses equal to or greater than a minimum number of processescalculated by the following Relational Expression 1:N=ROUNDUP{T/[(60/SPM)×(f/100)]}  [Relational Expression 1] in RelationalExpression 1, N represents a minimum required number of processes fromthe forming step except for the notching step and the blanking step, SPMrepresents the number of strokes per minute (SPM) of the press, frepresents a percentage (%) of a holding time in the vicinity of abottom dead center in one stroke, T represents a value calculated fromthe following Relational Expression 2 when 0.8≤t≤1.5, and represents avalue calculated from the following Relational Expression 3 when 1.5≤t,and ROUNDUP represents a value obtained by rounding up the number belowthe decimal point of the calculated value within { },T=t  [Relational Expression 2]T=5×t−6  [Relational Expression 3] in Relational Expressions 2 and 3, tis a thickness of the material, and a unit thereof is mm.
 10. The methodfor manufacturing a hot press formed member for a multistage process ofclaim 9, wherein f satisfies the following Relational Expression 4 when0.8≤t≤1.5 is satisfied, and satisfies the following RelationalExpression 5 when 1.5≤t is satisfied:0.8×t+2.6≤f  [Relational Expression 4]4.4×t−2.8≤f  [Relational Expression 5] in Relational Expressions 4 and5, t is a thickness of the material, and a unit thereof is mm.
 11. Themethod for manufacturing a hot press formed member for a multistageprocess of claim 1, wherein when the material is put into the processingapparatus, a pressing slide is lowered to a pressing bottom dead centerto process the material, the material stays in the vicinity of thepressing bottom dead center during lifting of the pressing slide, andthen, when the pressing slide reaches a point that does not interferewith the transfer of the material, the material positioned in thevicinity of the pressing bottom dead center is lifted utilizing positioninformation of the pressing slide.
 12. The method for manufacturing ahot press formed member for a multistage process of claim 1, wherein themethod includes a blanking step of obtaining a blank material separatedfrom the strip by cutting a part of the heated strip material put intothe processing apparatus using the blanking mold, and a tongs-shapedtransfer unit is used when the material is transferred from a mold in aprevious step to the vicinity of a mold in a subsequent step based ontwo molds adjacent to each other in a transfer direction of the materialin the one press.
 13. An apparatus for manufacturing a hot press formedmember for a multistage process, the apparatus comprising: a supply unitfor continuously supplying a strip material; a heating unit for heatingthe strip material; a processing unit including a processing apparatusin which a plurality of molds including one or more molds of a notchingmold and a blanking mold, a forming mold, and a trimming mold aremounted on one press; and a transfer unit for transferring the stripmaterial heated in the heating unit to the processing unit.
 14. Theapparatus for manufacturing a hot press formed member for a multistageprocess of claim 13, wherein the processing apparatus further includes atemperature control unit for controlling an initial temperature of oneor more molds of the notching mold and the blanking mold to 400° C. orhigher, and the temperature control unit is provided between the one ormore molds of the notching mold and the blanking mold and the press. 15.The apparatus for manufacturing a hot press formed member for amultistage process of claim 13, wherein a push bar operated by a springis provided as a structure protruding from a surface of an upper mold ofone or more molds of the notching mold and the blanking mold in contactwith the strip material.
 16. The apparatus for manufacturing a hot pressformed member for a multistage process of claim 13, wherein the onepress includes a pressing slide and a press bolster, and a materialposition control unit in the form of a cylinder is provided on a surfaceon which the mold on the press bolster is provided.
 17. A hot pressformed member manufactured by the method for manufacturing a hot pressformed member for a multistage process of claim 1, wherein the hot pressformed member has an under-cut shape, and a tensile strength of the hotpress formed member is 1,300 MPa or more.